Conventional industrial control systems are based on wired technologies like fieldbus systems and industrial Ethernet. Wireless technologies provide a low cost alternative with many additional benefits such as fast installation and commissioning, higher flexibility, ease of maintenance and increased troubleshooting capabilities. Recent advances in wireless technologies have led to the use of multi-hop wireless networks for (open-loop) monitoring of large-scale industrial systems. However, the use of multi-hop wireless networks for control applications is still at a nascent stage.
Multi-hop control networks are representative of wireless networked control systems in which control and feedback signals are exchanged in the form of information packets over a shared wireless medium, thereby closing a global control loop. The closed-loop control typically takes place between a controller and a spatially distributed system of sensors and actuators. Closed-loop control can also occur between controllers. This is referred to as control-to-control communication. Multi-hop control network find broad range of applications in legacy and emerging industrial systems, such as discrete manufacturing, process control, collaborative remote operation, remote control of automated guided vehicles, platooning of vehicles, and virtually coupled train systems—among many others.
Realizing closed-loop control over multi-hop wireless networks becomes particularly challenging with state-of-the-art wireless technologies. This is mainly due to the limitations of existing technologies in meeting the stringent requirements of closed-loop control applications. Typically, closed-loop control over wireless demands connectivity with very high reliability and very low latency. This is because packet losses and delays, which are dominant in wireless environments, have a detrimental effect on the stability of the control loop. Closed-loop control also involves bi-directional communication with cyclic traffic patterns. The cyclic information exchange demands highly deterministic connectivity which implies that the communication latency (between cycles) must have a very low variance. Besides, due to the presence of a large number of sensor and actuators, the requirement for highly scalable connectivity naturally arises.